The present invention relates to a pulse detector, and more particularly to a pulse detector comprising a pulse counter and an electrocardiac potential detector for detecting an electrocardiac potential induced on the skin of a person and for counting the pulse rate.
Generally the electrocardiac potential induced between both arms of a person consists of P wave, Q-R-S wave and T wave which develop periodically. Of these the variation in the electric potential V.sub.p-p of the Q-R-S wave is largest, i.e., between about 0.2 mV and 1.0 mV. Thus the method of detecting the Q-R-S wave is generally applied. Further, the electric noise at 50 Hz or 60 Hz induded on the skin of the person from the outside overlaps the electrocardiac potential. In counting the pulses, accordingly, it is necessary to eliminate a large noise level for accurately sensing minute signals.
A conventional circuit system for counting the pulses by detecting the electrocardiac potential is shown in a circuit block of FIG. 2. An electrocardiac potential sensing electrode 1 is connected to an input terminal of an amplifier 2. Faint electrocardiac potentials are amplified by the amplifier 2.
An output signal of the amplifier 2 is fed to a band-pass filter 3. A center frequency f.sub.0 of the band-pass filter 3 is designed at the frequency of the Q-R-S wave, about 20 Hz; while the P-wave and T-wave, and the 50 Hz and 60 Hz noises are attenuated or eliminated. An output signal of the band-pass filter 3 is fed to a low-pass filter 4. The noise, which is not eliminated by the band-pass filter 3 but transmitted to the low-pass filter 4, is eliminated by the low-pass filter 4. An output signal of the low-pass filter 4 is fed to a comparator 5. The comparator 5 detects only the Q-R-S wave of more than a given electric potential level and produces a pulse signal at an output terminal. The output signal of the comparator 5 is fed to a counter 6. The counter 6 counts the interval T (second) between the pulse signals and performs the operation of 60/T, and produces the operated result. The operated result indicates the pulse rate of the person. The output of the counter 6 is fed to a display 7. Thus the pulse rate is displayed on the display such as liquid crystal through a driver.
The electrocardiac potentials of persons differ according to their physical constitution and age. The induced electrocardiac potentials as well as noises differ even in the same person at different times because of the difference in a circumstances, such as humidity. Accordingly, the conventional type counter is disadvantageous in that the counting may sometimes stop by change of the person or even by change of the circumstances, and misoperation may easily occur by noise. Conventionally, a method of detecting even a small electrocardiac potential has been tried, in which the amplification degree of the amplifier 2 and the sensitivity of the comparator 5 are set at a high level on the assumption that these phenomena would occur. However, the pulse could not always be detected.